Magnetic quantum oscillations in the normal and superconducting state of nonmagnetic borocarbides

We report on de Haas-van Alphen (dHvA) investigations of the non-magnetic borocarbides YNi₂B₂C and LuNi₂B₂C. The measurements were carried out on high-quality single crystals by the torque method in magnetic fields up to 30 T. In the normal state the Fermi-surface topology has been analysed. In comparison with band-structure calculations several deviations are determined. Below the upper critical field, B_c2, in the vortex state an additional damping of the dHvA amplitudes appears. This damping is caused by the evolution from of the superconducting gap parameter. Concerning these topics we observe strong differences between differently grown crystals. The crystals produced by a zone-melting method show an abrupt vanishing of the dHvA signal at the upper critical field B_c2. In contrast, the flux grown crystals display a broader phase transition combined with a continuous decrease of the dHvA amplitude.